Android - Change a portion of a canvas' alpha on a SurfaceView using drawRect - is it possible?

http://stackoverflow.com –
Is it possible alter a portion of a the canvas' alpha to change its opacity? What I'm kind of looking for is something like this:
public void onDraw(Canvas canvas){
canvas.drawARGB(255,0,0,0);
.
.
//Other images ( Canvas.drawBitmap() ) goes in here
.
.
Paint paint = new Paint();
paint.setAlpha(100);
canvas.drawRect(20,50,40,100, paint);
}
I am trying to create a game where when the player comes across a tall obstacle, part of his body (For example, just his feet) can be seen through the obstacle with an alpha of 100, depending on how covered his body is with th
(HowTos)